You can set AMM and database automatically select the correct value for SGA and PGA:
http://docs.oracle.com/cd/E11882_01/install.112/e24321/inst_task.htm#BABBBHJH

Specify Configuration Options:
Provide the following configuration information, and click Next:
Memory:
The Enable Automatic Memory Management option is selected by default. This option enables the database to automatically distribute memory between SGA and PGA. If you deselect this option, then the SGA and PGA must be sized manually.

In addition to AMM, you should consider whether it is shared server or dedicated server.

the UGA is in the PGA in dedicated server
the UGA is in the SGA (large pool if configured, shared pool otherwise) in shared server

but the UGA is the UGA - your user global area, your session specific information, that data that must be available for the life of your session (like plsql variable values, cursor state information, etc)